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wrote on 5/28/2017 11:35 AM:
On Saturday, May 27, 2017 at 6:57:12 PM UTC-4, Ned Simmons wrote:
On Sat, 27 May 2017 12:20:00 -0700 (PDT), wrote:

On Saturday, May 27, 2017 at 2:26:40 PM UTC-4, Steve W. wrote:
wrote:
If a centrifugal pump with a maximum pressure of, say, 10 psi is
supplied with water at 80 psi will the water pressure coming out of
the pump be 90 psi? I think the pressure will be 90 psi. Am I wrong?
Thanks,
Eric

IF the volume of water remains constant the pressure coming out will be
at most 80 psi. if the pump is designed to produce 10 psi. It may be
lower depending on the size of the housing and the restriction the
impeller creates. Say your input side is 2" and the pump can produce 10
psi. at zero head pressure out of a 1.5" outlet.

Feed that pump with an 80 psi head pressure and the pump won't add any
pressure because it cannot pump faster than the water is already flowing
through it.

That's exactly what I thought, but Jim's reference to multi-stage pumps threw me. Since water isn't compressible, I don't see how the multi-stage pumps work. For gas, no problem, but I don't get it for liquids.

The pumps in series business is confusing the issue. The output of the
first (centrifugal) pump in the chain is far from an ideal pressure
source. The original question was about a pump with a constant inlet
pressure, either 0 psig or 80 psig. Flow wasn't specified, but as long
as the flow is constant for both inlet conditions, the delta P across
the pump will be the same. In other words, the pump will increase the
pressure by 10 psi in both cases.

This is where I have trouble. Assuming these are regular centrifugal turbines, the outlet of the first stage is fed into the axis of the second stage. The pressure from the first-stage outlet is retained at the second-stage inlet, but from there it feeds into the whirling blades of the second stage, the outlet volume of which is LARGER than the inlet volume between any two blades.

Pressure, thus, is converted to velocity. Unless the machine *compounds* the velocity at each stage, I don't see how it works. And, in order to compound velocity by a factor of, say, three, either the shaft driving the stage either has to be turning at (square root of 3) times that of the first stage, or the the second stage has to have a completely different scroll design.

But you can carry that only so far. Go to three stages, or four, and the shaft rotational speeds become outrageous, or the scroll design does.

Obviously, I'm missing something here, but I haven't yet seen what it is.

Ed Huntress

A centrifuge pump creates pressure by spinning fluid away from the
center (hence "centrifuge").

You are confusing yourself by of processing too much information at one
time. To make it easy for you, let's consider the output valve is closed
(the system is creating pressure but not expelling anything).

The first pump supplies the pressurized fluid to the second pump, the
second pump spins the pressurized fluid away from the center to add more
pressure to the housing wall. The pressure gauge in the second stage
should register more pressure than the first stage.

Does this help you understand better now, Ed?